This application claims the priority of German Application No. 197 36 560.4, filed Aug. 22, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a process for producing a body having a porous matrix from at least one recrystallized material, by shaping a raw material batch which contains a raw material powder and then sintering. The invention also relates to a body which can be produced by the process. The invention further relates to a process for producing a fiber-reinforced body having a porous matrix and to a body which can be produced according to the process.
Sintered silicon carbide (SSiC) is obtained from compact grained SiC particles with sintering additives for activation and carbon additives for dense sintering. Additives may be, for example, additions of boron and carbon or aluminum and carbon. A batch of raw material is shaped producing a green body, and the green body is sintered without pressure at approximately 2,000.degree. C. A dense product with more than 96% theoretical density is obtained. However, densification during sintering results in shrinkage.
Recrystallized materials, such as recrystallized silicon carbide (RSiC) and recrystallized boron carbide (RB.sub.4 C) are generally known. Recrystallized aluminum nitride (RAlN), recrystallized titanium carbide (RTiC) and recrystallized boron nitride (RBN) are also known. They are generally obtained by sintering batches of raw material which contain raw material particles and optionally additional substances.
Recrystallized silicon carbide is the best known recrystallized material. In comparison to conventional SSiC, it advantageously does not shrink during sintering. Recrystallized silicon carbide is obtained by sintering silicon carbide particles, preferably with bimodal grain distributions. Processes of this type are known from U.S. Pat. No. 2,964,823, German Patent Document DE-OS 2 837 900 and German Patent Document DE 31 49 796. The average particle size of the coarse grain is on the order of about 100 .mu.m. The size of the fine grain is approximately 1.5 to 8 .mu.m. These particles are sintered to a porous body at temperatures of above 2,000.degree. C. No sintering additives are required. The sintering mechanism is known. The presence of free carbon is harmful to the consolidation of the structure because it hinders or even prevents the sintering mechanism. No densification takes place during sintering and no shrinkage. A pure undensified porous silicon carbide is obtained.
Silicon carbide is a constituent of fiber-reinforced ceramics, which are preferably produced by the polymer route, by chemical vapor infiltration (CVI) or by liquid silicating. During liquid silicating of fiber-reinforced ceramics, a carbon matrix in contact with liquid silicon is converted to silicon carbide. The carbon matrix to be converted is normally produced by the pyrolysis of resins with a high carbon yield.
The production of fiber-reinforced ceramics by silicating C-matrices--mainly carbon-fiber-reinforced matrices--is known per se. Fibers are mixed with the resin, and optionally with additional substances, shaped and hardened. A resulting green compact is pyrolized. A porous fiber-reinforced silicated carbon body results.
Three problems are usually encountered in the process. First, an irregular pore distribution is usually observed, and very large pores or gaps are formed. During liquid silicating, silicon will fill the pores and remain. Remaining silicon can not be converted to silicon carbide.
Second, the resin which supplies carbon creates relatively massive irregular carbon matrix regions. During the reaction with liquid silicon, a silicon carbide layer forms on the surface around larger C-regions. The resulting approximately 5-10 .mu.m thick boundary layer prevents further conversion of the carbon enclosed by it. Conversion is only possible by way of diffusion processes over the boundary layer. If it takes place at all, it takes place very slowly. The carbon is therefore not completely converted.
Third, the resulting pore structure is not continuously open, or pore ducts may be closed by the converting reaction. Regions therefore exist into which liquid silicon cannot penetrate. In this case, no conversion of the carbon matrix can take place.
In order to provide a remedy, it has been attempted to create a crack structure into which the liquid silicon can penetrate, by way of, for example, pyrolysis of the resin. Pyrolysis creates an artificial duct porosity or open-pore condition. However, conversion to silicon carbide takes place only in the ducts.
Other attempts have been made to influence pore structure by resin additions. Complete carbon matrix conversion has not been achieved in these cases either.
In the case of the known process, fiber-reinforced silicon carbide ceramics are therefore obtained in which there remains residual carbon content and residual silicon content. Such ceramics, however, are not suitable for high-temperature applications in an oxidizing atmosphere because the residual carbon decomposes in air starting at approximately 400.degree. C. For applications above 1,400.degree., there must also not be any free silicon because silicon starts to melt at these temperatures.
It is therefore an object of the invention to provide a process and products of the above-mentioned type by means of which, in a simple and reasonably priced manner, bodies can be obtained which have a regular pore structure, particularly fiber-reinforced silicon carbon ceramics, in which the silicon as well as the carbon are completely converted to silicon carbide.
This and other objects and advantages are achieved by the process wherein a raw material powder is used during shaping and sintering which contains a fine grain fraction of an average grain size of at most approximately 2 .mu.m, and a coarse grain fraction of an average size of approximately 1.5 .mu.m to approximately 30 .mu.m, and wherein the sintering process is carried out at a temperature of at most about 1800.degree. C. The process permits the production of recrystallized ceramic materials having a defined, particularly fine and regular pore structure. As a result of a lowered sintering temperature, the process and product are more reasonably priced and suitable for industrial applications.
This object is also achieved by producing a raw material batch which contains a raw material powder having a grain size distribution with a fine grain fraction of an average grain size of at most about 2 .mu.m and a coarse grain fraction of an average grain size of about 1.5 .mu.m to approximately 30 .mu.m, mixing reinforcing fibers with the raw material batch, shaping the raw material producing a green body, sintering the green body at a temperature of at most about 1,800.degree. C. so that a fiber-reinforced body is formed having a porous matrix made of a recrystallized ceramic material, impregnating the fiber-reinforced body with a carbon-supplying substance and converting the carbon-containing substance to carbon so that a fiber-reinforced body is formed which has a carbon-containing porous matrix. This process permits adjusting of the structure and of the porosity of porous matrices such that a liquid infiltration medium, such as a silicon or carbon precursor, can homogeneously penetrate the whole body.
The object is also achieved with the products resulting from either of the above two processes.
The processes according to the invention are based on the recognition that problems, as they occurred in the previously mentioned processes, are the result of matrix shrinkage during pyrolyzing.
Although it is known that recrystallized materials, such as recrystallized silicon carbide, exhibit no shrinkage during sintering at lower temperatures, sintering temperatures had heretofore been much too high for using this material for production of fiber-reinforced ceramics. Conventional ceramic reinforcing fibers may be subjected to maximum temperatures of 1,600.degree. C. Fibers still under development, such as SiBNC fibers, may possibly be able to be subjected to higher temperatures in an inert atmosphere, but will still have temperature maximums of 2,000.degree. C. The high-temperature resistant C-fibers are also stable only under inert conditions.
It was found that, when particularly fine particles are used, the sintering temperature of recrystallized ceramic materials, such as silicon carbide, is considerably reduced, which was surprising and not to be expected. A certain grain size distribution is therefore required; the substance to be sintered must contain a fraction of particularly fine grains. This fine grain fraction sublimes at low temperatures and is deposited on energetically favorable regions, such as contact points of the coarser grains, resulting in consolidation without shrinkage.
The sintering temperature can therefore be controlled and adjusted in a targeted manner by way of grain size distribution. Sintering temperatures of below 2,000.degree. C., particularly below 1,800.degree. C. and below 1,400.degree. C., can be achieved.
By defining the grain size and grain size distribution, pore structure can also be adjusted. Thus, a defined, uniform, open-pore structure can be obtained. Subsequently, the pores can be filled, for example, with carbon or carbon precursors. During the production of fiber-reinforced silicon carbide ceramics, the homogenous pore structure can be controlled such that later a complete conversion of carbon with liquid silicon can take place. This can occur, for example, with offered carbon for filling the pores.
This eliminates the disadvantages of the prior art mentioned above. First, during production of fiber-reinforced silicon carbide ceramics, there remains no residual carbon content or residual silicon content. In addition, the structure is absolutely uniform; fibers are protected because of low sintering temperatures; and low temperatures result in cost reduction.
For grain size distribution, a selection can be made between several possibilities. Grain size may be monomodal with a broader grain distribution, but also bimodal with a fine-grain fraction and a coarse-grain fraction. A trimodal construction with a fine, a medium coarse and a coarse grain is also possible. In all cases, it is important that a fine grain fraction is present. If a high packing density is desired, a bimodal grain structure is preferably used.
The upper limit of the grain size of the fine grain fraction is approximately 2 .mu.m. The lower limit is small enough so that it cannot be reliably measured. A preferred grain size distribution is between approximately 0.25 .mu.m and 1 .mu.m. Usually expenditures of temperature and time during sintering are lower with smaller grain size of fine grain fraction.
In the case of a monomodal fine grain structure, the grain size ranges are preferably at a d.sub.50 value of less than approximately 10 .mu.m, preferably about 1 .mu.m.
Grain size of the coarse grain fraction is at approximately 1.5 .mu.m to 30 .mu.m and is independent of grain size of the fine grain fraction. Grain sizes of approximately 2 .mu.m to 5 .mu.m are preferred. When producing fiber-reinforced ceramics, grain size must be coordinated with fiber diameter.
Numerous possibilities exist for shaping the green body. Production by slip casting is preferred. A pressure diecasting or a film casting, extrusion processes, injection molding, isostatic pressing, axial pressing or RAM pressing and comparable processes may be used. With long fibers or fibrous tissue, known winding and depositing techniques also may be used in which the fibers or the tissue are dipped into a slurry of the raw material batch and are then wound or placed or pressed or otherwise processed.
For adjusting pore size and pore structure, in addition to grain size and grain distribution, shaping processes for producing the green body must also be included. During manufacturing of fiber-reinforced bodies, the matrix is also placed in fiber bundles and holds ducts open. The fibers are embedded completely into the porous matrix.
The green body, which may be provided with reinforcing fibers, is sintered at temperatures of up to approximately 1,800.degree. C., preferably approximately 1,400 to 1,800.degree. C. An open-pored porous body is obtained which has a uniform pore structure and can be infiltrated well. For producing fiber-reinforced silicon carbide ceramics, the porous body is impregnated with carbon or a carbon precursor. Carbon can be controlled by the type of precursor (by way of its carbon yield, or the degree of dilution of the carbon or of the precursor in solvent or a suspending agent). Silicating takes place under conventional conditions. In the process, shrinkage of the shaped body or of the composite does not take place in any process step.
For producing RSiC, in the case of the silicon carbide batches on which the invention is based, it is particularly advantageous that a carbon fraction be used which corresponds to silicon dioxide contained in the batches. The specific surface is always coated with a thin layer of silicon dioxide required for hardening. Heretofore, for producing recrystallized silicon carbide, carbon contamination hindered the sintering process. When a particularly fine grain is used, a particularly large specific surface is obtained, such as, approximately 35 m.sup.2 /g. Thus, a relatively high silicon dioxide content will be present which permits a corresponding carbon fraction without hindering hardening.
For producing the fiber-reinforced body, all known reinforcing fibers are suitable, including whiskers or platelets. These include carbon fibers, as well as oxidic and non-oxidic ceramic fibers, such as SiBNC fibers. These fibers may be short fibers as well as continuous fibers, such as long fibers or laid fibers.
For the recrystallized material, recrystallized boron carbide, recrystallized aluminum nitride and recrystallized titanium carbide are also suitable in addition to recrystallized silicon carbide.
The porous matrices may be mixed with a suitable metallic or non-metallic substance. Examples of non-metallic substances are carbon precursors or glass meltings. By the infiltration or reaction infiltration of these porous matrices, completely dense bodies can be produced or composite bodies with a completely dense matrix.
The pores may also only partially be treated, for example, with pure carbon (for example, for filters, such as gas filters).
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.